The reactors that power Britain's naval submarines are "potentially vulnerable" to fatal nuclear accidents because they fail to meet modern safety standards, according to a heavily censored report declassified by the Ministry of Defence.

The MoD's senior nuclear safety expert warns that the safety of submarine reactors compares "poorly" with that of nuclear power stations, and there could be dangerous leaks of radioactivity. There was also a risk of "multiple fatalities" from submarines failing to surface, he says.

Yet the reactors are being installed in a new £10bn fleet of Astute submarines. And they are still under consideration for the submarines due to replace those that carry Trident nuclear missiles from 2028.

The report, released under freedom of information law and passed to Channel 4 News, also reveals that arguments over the reactor designs have delayed decisions on replacing Trident by 18 months, and added more than £260m to the bill.

The report is a submission made to the Defence Board, a senior decision-making body within the MoD, in November 2009 about the status of plans to replace Trident submarines. It includes a damning assessment of the safety of current submarine reactors by the MoD's senior nuclear safety regulator, Commodore Andrew McFarlane.

"Current UK practice falls significantly short of benchmarked relevant good practice," he warns. The pressurised water reactors that run submarines are "potentially vulnerable to a structural failure of the primary circuit".

This could cause "a release of highly radioactive fission products" and "a significant risk to life to those in close proximity and a public safety hazard out to 1.5km from the submarine", McFarlane says.

"Current designs of UK and global civil power plants have systems for safety injection of coolant into the reactor pressure vessel head and passive core cooling systems," he adds.

"UK submarines compare poorly with these benchmarks, with the ability to tolerate only a structural failure equivalent to a …" Unfortunately the rest of the sentence, along with most of the following two pages, are blacked out in the released document.

McFarlane, who retired as head of the MoD's internal Defence Nuclear Safety Regulator in 2009, also suggests reactor problems could stop submarines from surfacing. There was a "risk of multiple fatalities resulting from loss of depth control", he says.

The MoD is also trying to decide on the type of reactor to drive the submarines destined to replace the Vanguards. The declassified MoD document discloses that there are several options still under consideration.

One, known as PWR2, is "essentially" similar to the Astute class, while another, PWR2b, has "significantly modified systems to improve platform safety and survivability". A third, PWR3, is described as a "new propulsion plant based on a US design but using UK reactor technology".

Uncertainty over which reactor to choose has delayed a crucial investment decision about Trident, known as "initial gate", by at least 18 months. The decision was originally due to be made in September 2009, but has still not been announced by the MoD.

The delay incurred "additional funding" of nearly £261m, the document says. It also meant that "substantial interest can be expected from parliament and elements of the media" that would require "presentational handling".

John Large, the consulting nuclear engineer who helped oversee the salvage of the stricken Russian submarine, Kursk, after it sank in August 2000, said the document revealed "very serious shortcomings" in the present generation of submarine reactors.

"These include doubts about the survivability of the submarine after a nuclear reactor malfunction, lack of a passive shut-down system, and strong hints that the reactor plant could fail when subject to what should be tolerable levels of hostile action."

John Ainslie, the co-ordinator of the Scottish Campaign for Nuclear Disarmament, who obtained the document from the MoD, called on the government to scrap the Trident replacement programme. "There is a serious risk that there will be a nuclear accident which will expose the crew and members of the public to lethal levels of radiation," he said.

An MoD spokesman said: "The MoD takes nuclear safety very seriously. All of our nuclear reactors meet the strict safety standards set by the DNSR and we continuously look for areas of improvement."

• This story is to be broadcast on Channel 4 News at 7pm on Thursday 10 March.